We hear normal referenced in weather reports and climate summaries daily, but what exactly does it mean? Unlike other parts of our lives, normal has a very strict definition in meteorology. Normal means average, but not just any average – it’s a 30 year average. Currently, normal refers to the 30 year average over the period from 1981 to 2010, including those years.
In my previous blog about drought, I referred to a normal amount of rainfall for RDU International Airport over the course of a year. That normal is the average of annual rainfall over that thirty year period. To get an annual average, you add the rainfall reported at a specific spot over those three decades and then divide the sum by 30. Our average annual rainfall at RDU is 43.34 inches, but that doesn’t mean that RDU gets that exact amount each year. Some years, perhaps the years in which hurricanes affect the area, we get much more. Other years we receive much less. Still, on average, we see about 43 inches of rain annually.
The same rules apply to high and low temperatures, snowfall, and just about any other weather product you might want to track on a daily, weekly, monthly, seasonal, or annual basis.